1. Field of the Invention
In many types of apparatus, such as two way radios, pagers, etc., specific signals are utilized to establish a communications link between two remote pieces of equipment. One common type of specific signal utilized includes one or more low frequency tones. It is, therefore, a necessity to include circuitry in these pieces of apparatus which will recognize specific signals or tones and provide an output signal when the correct signals or tones are received. This output signal is then used to activate audio or visual indicators, turn on receivers, etc.
Further, in some instances it is desirable to send the tones or specific signals along with audio or data signals. To do this a portion of the audio or data is notched out and the tones or specific signals are multiplexed into the notch. In these instances it is imperative that the notch remove a very small amount of the audio or data, but still include enough tone or specific signal to be detected.
2. Description of the Prior Art
In prior art devices, circuitry capable of recognizing specific signals or tones includes mechanical vibrating devices (reeds or crystals) or electrical filters which allow only the desired signals or tones to pass. These prior art devices are effective but the signal or tone must be present for a relatively long period of time before recognition or detection can occur. Further, the signal or tone must be monitored continuously by the prior art devices for the relatively long period of time.